Estimates Of Cycling Efficiency At Low Power Outputs

Efficiency at low power outputs, comparable to daily living activities, is relevant to understanding obesity. Calculating muscular efficiency has remained controversial due to the difficulties of isolating the energy expenditure associated with the skeletal muscle mass performing mechanical work. While subtracting baselines (e.g. metabolic rate at rest or during zero load cycling) is common, these subtractions are problematic. PURPOSE: To investigate the effects of different baseline subtractions on the calculation of efficiency during cycling, particularly at low power outputs. METHODS: 15 active subjects (age 18-35) completed a one-visit cycle-ergometer protocol. They performed 13, 5-minute trials including: seated rest, motor-driven (passive) cycling, no chain cycling, no load cycling, and cycling at low (10-30W), medium (40-60W), and high (80-120W) power outputs. All cycling trials were performed at 90 revolutions per minute (rpm) on a pan-loaded, mechanically braked cycle ergometer. During each trial, we continuously performed indirect calorimetry. RESULTS: VO2 during rest was 0.29, ±0.06 L/min, and nearly doubled during passive cycling (0.57, ±0.14 L/min). VO2 values for no chain (0.76, ±0.12 L/min) and no load (0.79, ±0.11 L/min) trials were not statistically different from each other. Thus, active muscle contraction constituted only about half of the increased cost of no load cycling vs. rest. Regression equation for VO2 vs. power output for 10-120 Watt trials was 0.007× + 0.752, (R2 = 0.994). Mean delta efficiency for low power outputs was 68.4%, compared to 36.3% and 36.9% for middle, and high power outputs. Means for gross (3.6%) and net (5.6%) efficiencies were rather low at low power outputs. Furthermore, at low power outputs, delta and work efficiencies demonstrated large individual variations exceeding theoretical values for muscular efficiency. CONCLUSION: No method of baseline subtractions gave satisfactory values for muscular efficiency at low power outputs.